1. Field of the Invention
The present invention relates to a spark plug.
2. Description of the Related Art
A spark plug used for ignition of an internal engines such as automobiles generally comprises a metal shell to which a ground electrode is fixed, an insulator made of alumina ceramics or the like, and a center electrode which is disposed inside the insulator. The insulator projects from the rear opening of the metal shell in the axial direction. A terminal fixture is inserted into the projecting part of the insulator and is connected to the center electrode via a conductive glass seal layer which is formed by a glass sealing procedure or a resistor. A high voltage is applied to the terminal metal fixture to cause a spark over the gap between the ground electrode and the center electrode.
However, under some combined conditions, for example, at an increased spark plug temperature and an increased environmental humidity, it may happen that high voltage application fails to cause a spark over the gap but, instead, a discharge called a flashover occurs between the terminal metal fixture and the metal shell, going around the projecting insulator. Primarily for the purpose of avoiding this flashover phenomenon, most of commonly used spark plugs have a glaze layer on the surface of the insulator. The glaze layer also serves to smoothen the insulator surface thereby preventing contamination and to enhance the chemical or mechanical strength of the insulator.
In the case of the aluminum insulator for the spark plug, a glaze of lead silicate glass has conventionally been used where silicate glass is mixed with a relatively large amount of PbO to lower a softening point. In recent years, however, with a globally increasing concern about environmental conservation, glazes containing Pb have been losing acceptance. In the automobile industry, for instance, where spark plugs find a huge demand, it has been a subject of study to phase out Pb-containing glazes in a future, taking into consideration the adverse influences of waste spark plugs on the environment.
Leadless borosilicate glass- or alkaline borosilicate glass-based glazes have been studied as substitutes for the conventional Pb-containing glazes, but they tend to be insufficient in mechanical strength. For example, in the process of producing spark plugs, they are liable to suffer chipping or delamination of the glaze layer upon conveying the insulators having formed thereon the glaze layer, in a state of being put side by side on a wire gauze, by the impact applied thereto during handling thereof.
An object of the invention is to provide spark plugs having glaze layers containing a less amount of Pb component, and having an excellent mechanical strength, in particular, impact resistance.